Captian

Swap the letter order a bit, to get closer to the pronounciation?

It’s grammatically incorrect here and makes my eye twitch.

You wouldn’t say “whom among us” so you also wouldn’t say “whomst among us.”

You can use it as an object of a verb or preposition though. Like:

“Whomst dost thou hunt?” Or “Those from whomst we flee doth terrify!”